The implantation of stents has become established as one of the most effective therapeutic measures for the treatment of vascular diseases. Stents have the purpose of performing a stabilizing function in hollow organs of a patient. For this purpose, stents featuring conventional designs have a filigree supporting structure comprising metal braces, which is initially present in compressed form for introduction into the body and is expanded at the site of the application. One of the main application areas of such stents is to permanently or temporarily dilate and hold open vascular constrictions, particularly constrictions (stenoses) of the coronary blood vessels. In addition, aneurysm stents are also known, which are used primarily to seal the aneurysm.
Stents have a peripheral wall with sufficient load-bearing capacity in order to hold the constricted vessel open to the desired extent and a tubular base body through the blood continues to flow without impairment. The peripheral wall is generally formed by a lattice-like supporting structure, which allows the stent to be introduced in a compressed state, in which it has a small outside diameter, all the way to the stenosis of the particular vessel to be treated and to be expanded there, for example by way of a balloon catheter, so that the vessel has the desired, enlarged inside diameter. As an alternative, shape memory materials such as nitinol have the ability to self-expand when a restoring force is eliminated that keeps the implant at a small diameter. The restoring force is generally applied to the material by a protective tube.
The stent has a base body made of an implant material. An implant material is a non-living material, which is used for applications in medicine and interacts with biological systems. A basic prerequisite for the use of a material as implant material, which is in contact with the body area when used as intended, is the body friendliness thereof (biocompatibility). Biocompatibility shall be understood as the ability of a material to evoke an appropriate tissue response in a specific application. This includes an adaptation of the chemical, physical, biological, and morphological surface properties of an implant to the recipient's tissue with the aim of a clinically desirable interaction. The biocompatibility of the implant material is also dependent on the temporal course of the response of the biosystem in which it is implanted. For example, irritations and inflammations occur in a relatively short time, which can lead to tissue changes. As a function of the properties of the implant material, biological systems thus react in different ways. According to the response of the biosystem, the implant materials can be divided into bioactive, bioinert and degradable/resorbable (referred to here as biocorrodible) materials.
Implant materials comprise polymers, metallic materials, and ceramic materials (as coatings, for example). Biocompatible metals and metal alloys for permanent implants comprise, for example, stainless steels (such as 316L), cobalt-based alloys (such as CoCrMo cast alloys, CoCrMo forge alloys, CoCrWNi forge alloys and CoCrNiMo forge alloys), technical pure titanium and titanium alloys (such as cp titanium, TiAl6V4 or TiAl6Nb7) and gold alloys. In the field of biocorrodible stents, the use of magnesium or technical pure iron as well as biocorrodible base alloys of the elements magnesium, iron, zinc, molybdenum, and tungsten are proposed. The present invention relates to non-biodegradable implant materials, in particular cobalt-based alloys.
Stents must have the ability to tolerate large plastic elongation and maintain the size or diameter thereof when they are expanded. In general, the ideal stent should:                have a low profile; this includes the suitability of being crimped onto a balloon catheter;        exhibit good expansion properties; when the stent is introduced in the lesion and the balloon is inflated, the stent should uniformly expand so as to adapt to the vessel wall;        have sufficient radial strength and negligible recoil; once the stent has been placed, it should withstand the restoring forces of the atherosclerotic vessel wall and not collapse;        have sufficient flexibility to bending; the stent can thus also be delivered through vessels and stenoses having smaller diameters;        have adequate radiopacity or MRI compatibility; the medical staff can thus assess the implantation and position of the stent in vivo;        have low thrombogenicity; the material should be biocompatible and in particular prevent the deposition and agglutination of platelets;        have the option of releasing active agents; this is used in particular for preventing restenosis.        
The requirements address in particular the mechanical properties of the material of which the stent is produced. The classic 316L, MP53N and L-605 materials that are used for constructing balloon-expandable stents have mechanical disadvantages which restrict the freedom in stent design development and in use:    (i) insufficient (ultimate) tensile strength UTS and elongation at fracture E            As a result, the collapse pressure and radial strength are lower, so that thicker stent struts are required, which causes a larger loss of lumen during implantation, delays healing (endothelialization) into the vascular wall, and restricts the freedom in the geometric stent design development. Thicker struts additionally make the stent more rigid, which reduces the flexibility around bends.            (ii) yield tensile strength YTS too high            This result in high elastic rebound, which worsens the crimpability, leads to a thicker crimp profile and causes higher recoil (loss of lumen due to expansion).        
Moreover, the biocompatibility of the material must be ensured. Nickel has been repeatedly listed as causing allergies or local and systemic incompatibilities. A need therefore exists for nickel-free materials for medical use.
DE 197 04 530 A1 describes a nickel-free, austenitic cobalt-based alloy for avoiding allergies in various objects of use, including implants, having high corrosion resistance and good formability. Here, nickel is replaced as the stabilizer for the austenitic state by adding titanium and/or niobium (together 4 to 6% by weight). The alloy additionally contains Cr (10 to 18% by weight), Fe (5 to 20% by weight), and Mo and W (together 4 to 8% by weight, with the content of W being half that of Mo). Moreover, the alloy can contain Cu (0 to 2% by weight), Mn (0 to 3% by weight), Al (0 to 3% by weight), Si, (0 to 1% by weight), C (0 to 0.1% by weight) and N (0 to 0.1% by weight). The disadvantages of this alloy are insufficient ductility and, more particularly, insufficient radiopacity for the use as a stent material. In addition, Cu and Al are not considered to be biocompatible.
U.S. Pat. No. 3,865,585 describes a nickel-free cobalt-based alloy comprising Cr (26 to 31% by weight), Mo (4 to 6.5% by weight), Si (0 to 2% by weight), Fe (0 to 1% by weight), B (0 to 0.5%) by weight, C (0 to 0.5% by weight), N (0.15 to 0.5% b weight), with the cumulative content of C and N not exceeding 0.7% by weight. However, at less than 20%, the ductility of the alloy is very low and not suited for stents.
DE 36 24 377 A1 proposes a cobalt-based alloy having the following composition for medical implants and fixed dental prostheses: Cr (15 to 24% by weight), Fe (2 to 15% by weight), Mo (3 to 10% by weight), N (0 to 0.05% by weight) and C (0 to 0.05% by weight). At approximately 10%, the ductility is very low and not suited for stents.
A continued need thus exists for a nickel-free metallic implant material that has sufficiently high ductility and is suited for the production of stents.